I The study of plant morphology and morphogenesis has developed extensively with the advent of recombinant technology. Investigators have identified individual homeotic genes that act either alone or in concert with each other to determine structures in the developing plant. Variations in plant morphology have been noted through the ages, and studies have revealed that mutations of critical homeotic genes may account for such structural aberrations. In many instances such changes may be prompted by response of the plant to environmental stress. The bulk of these investigations have sought to determine both the manner in which such aberrations result and possible avenues for application of such aberrations either to develop new plant strains and structures, or alternatively, to correct the defect caused by the mutation and to assure uniform and normal plant growth.
More particularly, plants adapt to environmental stresses through altered growth patterns and physiological changes. These changes occur primarily at the tips of the plant, through the regulated division of cells in regions termed apical meristems. Meristems are first laid down in the embryo, but actively and continuously produce new organs in the germinated seedling and adult plant. The root apical meristem is a particularly suitable system for studying organogenesis because of its relatively simple organization and well characterized, predictable cell divisions (Dolan et al., 1993). Most of the root is a cylinder composed of concentric layers of four basic tissues (epidermis, ground tissue [cortex and endodermis], pericycle and vascular bundle from outside to in; FIG. 1). Anatomical studies and fate mapping trace the origin of each tissue type to a ring of cells at the root apex known as initials (Scheres et al., 1994). Initials produce two cells in every stem-cell division: the regenerated initial, and a daughter cell which differentiates as it is displaced from the initial by further rounds of division. Thus, growth in the root occurs by addition of cells at the tip. Surrounding the root apex is a set of protective cells which make up the root cap. The columella (central) root cap has its own set of initials which produce daughters downwards in the direction of the root tip. Lateral root cap shares a common initial with the epidermis. Together, the initials surround a set of approximately four central cells in Arabidopsis termed central cells, or the quiescent center (QC). The meristem is considered to comprise the QC, initials, and their rapidly dividing derivatives in the root tip.
The QC, at the heart of the mersitem, has been the focus of intense investigation for nearly two centuries. Early theories posited that central cells must divide rapidly to give rise to all tissues of the root (see Barlow, 1976). This idea was debunked by the English investigator Frederick Clowes, who fed 3H-thymidine to maize seedlings, and discovered that the population of central cells rarely enter S phase of the cell cycle (Clowes, 1961). Slow cycling time has since been described as an almost universal aspect of stem cells, which often accompanies self-renewal as a defining character (Morrison et al., 1997). Soon after the discovery of mitotic quiescence, levels of RNA and protein synthesis were determined to be very low in the QC (Clowes, 1961). Despite these properties, it was shown that occassional divisions could give rise to any cell of the root, and damaged meristems were regenerated by inducing QC proliferation (Feldman, 1976). This led Barlow to propose his Founder Cell Theory, in which QC cells were described as the ultimate source of all cells in the root (Feldman, 1984). Totipotency of QC cells was demonstrated through in vitro experiments, which proved that the QC is sufficient for regenerating an organized root in culture (Feldman & Torrey, 1976). Torrey (1972) looked beyond the role of the QC as a stem cell population by proposing a function in patterning. He argued that the QC served as a “template”, by which the meristem and root was patterned. Although a quiescent center has been detected in all plant species analysed to date (including Arabidopsis; Dolan et al., 1993), its function remains poorly characterized.
More recent insights into QC function have come from laser ablation studies. Ablation of a single QC cell results in the differentiation of contacting initials, as evidenced by accumulation of starch in the columella initial and a premature asymmetric division in the ground tissue (van den Berg et al., 1995). Thus, the QC must be responsible for a short-range signal that prevents the differentiation of surrounding stem cells. An additional, long-range “top-down” signal for differentiation was also detected in roots by laser ablation (van den Berg, 1997). In order to maintain its undifferentiated state, the QC must be refractile to this signaling from more mature cells. Given their function in continuous organogenesis, meristems must be able to balance stem-cell maintenance with differentiation. In the shoot apical meristem, stem cells in a central zone undergo cell divisions to produce daughters in a peripheral zone where organ primordia are initiated. Balance between these processes in the shoot has recently been shown to result from an autoregulatory loop between signaling molecules (Schoof et al., 2000). The homeodomain protein WUSCHEL is expressed in an organizing region just below the central zone, and is required for maintenance of stem cell fate (Mayer et al., 1998). WUS activates transcription of CLV3, a ligand which signals the CLV1 receptor kinase in the central and peripheral zones to inhibit transcription of WUS. Thus, as stem cells leave the central zone, they lose their ability to repress WUS, an activator of stem cell fate; these cells are replenished by the activity of WUS in the central zone.
Superficial similarities between the root and shoot meristems, in combination with the results of laser ablation, hint that a similar feedback regulation may occur between the QC and surrounding cells. Along these lines, the FASCIATA1 and 2 genes have recently been shown to affect both shoot and root apical meristem organization as a double mutant (Kaya et al., 2001). FAS genes encode proteins involved in replication-fork dependent nucleosome assembly, and are thus hypothesized to maintain stable epigenetic transcriptional states in the meristems. In fas double mutants, the WUS expression domain is expanded, leading to stem cell overproliferation and stem fasciation. Candidate targets in the root are few. Recently, the root-tip specific expression of CakAt1, a CDK-activating kinase, has been shown to be necessary for preventing differentiation of initials (Umeda et al., 2000). This is likely to be a downstream player of signaling or morphogenic processes.